Anatomy

The pulmonary circulation has its beginning in the right part of the heart. The blood that has supplied the organs with oxygen is now enriched with carbon dioxide and low in oxygen. This blood from the body is pumped via the right atrium and the right main chamber (= ventricle) into the truncus pulmonalis (= pulmonary artery).

The truncus pulmonalis branches out along the anatomy of the airways into a right and a left pulmonary artery. These branch and branch out into ever smaller vessels up to the so-called capillaries. They surround the many millions of pulmonary alveoli, which are filled with air.

The blood flows very slowly in the capillaries, as this is where the oxygen exchange between the alveoli and the capillaries takes place.The carbon dioxide is released through the thin walls of the capillaries and alveoli and exhaled through the air we breathe, while in return oxygen is absorbed into the bloodstream. The smallest veins, so-called venules, unite from the capillaries to form ever larger veins and transport the now oxygen-rich (= oxygenated) blood back to the heart. It now reaches the left part of the heart and is pumped from there via the aorta into the body’s circulation.

Changing the circulation at birth

Before birth, this pulmonary circulation is not needed because the fetus is supplied with the mother’s oxygen via the umbilical cord. The lungs are not yet ventilated. For this reason, there is an opening between the trunkus pulmonalis and the aorta, which is called the ductus arteriosus.

There is also a small hole between the right and left atrium (foramen ovale). With the first cry after birth, the pressure reverses as the lungs are ventilated. Now both the Foramen ovale and the Ductus arteriosus should close. If this does not happen, various problems of adaptation may arise in the newborn and possibly therapy or surgery to close the opening may be necessary.

What is the pressure in the pulmonary circulation?

The pulmonary circulation is part of the so-called low pressure system. The average pressure is between 0 and 15 mmHg. The low pressure system includes the capillaries, the veins, the right part of the heart, the vessels of the pulmonary circulation and the left atrium of the heart.

In the body’s circulation, however, as part of the so-called high-pressure system, pressures of between 70 and 120 mmHg prevail at rest. All vessels of the low-pressure system are characterized by a higher degree of elasticity than the vessels of the high-pressure system. This is due to the main task of the low pressure system – the intermediate storage of blood. If there is a shortage of blood and consequently an undersupply of blood to the organs, the volume of blood stored in the low pressure system can be used to initially ensure a supply to the organs.